Ultrastructural immunocytochemical localization of tyrosine hydroxylase in the neostriatum

The morphology and synaptic associations of dopaminergic axons in the n. caudate-putamen (neostriatum) of the adult rat brain are examined. Identification of dopaminergic axons is based upon the electron microscopic immunocytochemical localization of the catecholamine synthesizing enzyme, tyrosine hydroxylase. Immunoreactivity for the enzyme is detected in unmyelinated axons and axon terminals in serial sections collected throughout the neostriatum. The labeled terminals range from 0.1 to 1.5 micron in diameter and have peroxidase reaction product located around closely packed, round vesicles with a diameter of 40-60 nm. The tyrosine hydroxylase containing axon terminals constitute approximately 21% of the total number of terminals in the n. caudatus-putamen and include 3 types which differ in size and synaptic specializations. The most prevalent (82% of total), type I, is small (0.15-0.39 micron in diameter) and forms symmetric junctions with dendrites and dendritic spines. The other two terminal types (II and III) have a medium to large diameter (0.4-1.5 micron) and show either no membrane specializations or asymmetric junctions with dendrites. The axon terminals without observable membrane densities are occasionally oriented so as to suggest an association with dopaminergic and non-dopaminergic axon terminals. These findings indicate that while the dopaminergic terminals may form axoaxonic connections, the primary synaptic contacts are with dendrites of intrinsic neurons in all regions of n. caudatus-putamen.